1. Field of the Invention
This invention relates to a data transfer system for transferring time series digital data in a data packet at high speed.
2. Description of the Related Art
IEEE1394-1995 standard is proposed as an interface standard for putting time series digital data of audio signals, video signals, etc., into a data packet and transferring the data packet at high speed between electric machines such as audio machines, video machines, computers, etc.
However, in a data transfer system based on the IEEE1394-1995 standard, it is conceivable that the data to be transmitted becomes past data from the reference time at the transmission time in a transmitter, for example, because of change in the reference time in the system, a malfunction, etc., and in the party receiving the data, digital data cannot be reproduced at proper timing from the later received data packet over a considerable period of time.
It is therefore an object of the invention to provide a unit, when the data to be transmitted becomes past data from the reference time because of change in the reference time, a malfunction, etc., the unit for correcting the data to data based on the current time and transmitting the corrected data.
It is another object of the invention to provide a unit for enabling a receiver to deal with data for which the problem cannot be solved in a transmitter.
According to a first aspect of the invention, there is provided a transmission interface unit in a transmission system wherein time series data is divided into data groups and a data packet comprising reproduction specification time data specifying the time at which each data piece in the data groups should be reproduced, added to the data groups is transmitted on a transmission bus in a time division manner, the transmission interface unit comprising a cycle timer for counting the reference time of the home machine based on the reference time on the transmission bus, generation means for generating the reproduction specification time data, and addition means for dividing the time series data into data groups, putting into packets, and adding the reproduction specification time data to the data in the packets, characterized by rewrite means for rewriting the reproduction specification time data generated by the generation means when the reference time on the transmission bus changes.
In a second aspect of the invention, the transmission interface unit as set forth in the first aspect of the invention further includes determination means for determining whether the reproduction specification time data generated by the generation means is reproduction specification time data generated before or after the reference time on the transmission bus changes and supplying the reproduction specification time data generated before the reference time on the transmission bus changes to the rewrite means based on the determination result.
In a third aspect of the invention, in the transmission interface unit as set forth in the first or second aspect of the invention, the cycle timer comprises at least two registers each for temporarily retaining a reference time transmitted on the transmission bus, at least the reference time of the home machine set before the reference time on the transmission bus changes and the reference time of the home machine set after the reference time on the transmission bus changes are retained in the registers, when the reference time on the transmission bus changes, the cycle timer calculates a difference between the reference times retained in the registers and transmits the difference to the rewrite means, and the rewrite means rewrites the reproduction specification time data based on the difference.
In a fourth aspect of the invention, in the transmission interface unit as set forth in the second or third aspect of the invention, the determination means and the rewrite means are at the stage following the addition means.
According to the first aspect of the invention, when the reference time on the transmission bus changes, the rewrite means rewrites the reproduction specification time data generated before the reference time on the transmission bus changes. Thus, when the data to be transmitted onto the bus becomes past data from the reference time, the data can be corrected to data based on the current time and the corrected data can be transmitted.
According to the second aspect of the invention, the determination means determines whether the reproduction specification time data is reproduction specification time data generated before or after the reference time on the transmission bus changes and can supply the reproduction specification time data put into a packet before the reference time on the transmission bus changes to the rewrite means based on the determination result. Thus, only the data which is to be transmitted onto the bus and is past data from the reference time can be appropriately corrected to data based on the current time and the corrected data can be transmitted.
According to the third aspect of the invention, when the reference time on the transmission bus changes, the cycle timer calculates a difference between the reference times of the home machine retained in the registers and transmits the difference to the rewrite means, and the rewrite means can rewrite the reproduction specification time data based on the difference. Thus, only the data which is to be transmitted onto the bus and is past data from the reference time can be appropriately corrected to data based on the current time and the corrected data can be transmitted.
According to the fourth aspect of the invention, the determination means and the rewrite means are at the stage following the addition means, thus making it possible to correct the reproduction specification time data just before the data is transmitted onto the transmission bus.